High-efficiency broadband klystron amplifier of reduced length

ABSTRACT

A multi-cavity klystron is provided which utilizes a front stage wide band exciting cavity group consisting of one or more cavities and including at least one cavity coupled to an external reactance circuit. Means are provided for applying a voltage to accelerate the electron beam at least across the last drift tube interaction gap in said front stage wide band cavity group. A rear stage bunching cavity group comprised of one or more cavities is tuned to a frequency of few percent greater than the center frequency operating band and an output cavity is provided to ultimately receive and pass the electron beam, said output cavity being tuned substantially to the center frequency of the operating band. By selecting a perviance measured across the cathode and accelerating electrode, which is greater than the values heretofore utilized, the perviances of the succeeding gaps through which the electron beam successively passes may be accordingly reduced to realize a high conversion efficiency, typically of the order of 66.6%, without making the overall axial length of the klystron greater.

United States Patent 1 1 11 3,904,917

Ueda 1 Sept. 9, 1975 HIGH-EFFICIENCY BROADBAND Primary ExaminerSaxfield Chatmon, Jr.

KLYSTRON AMPLIFIER OF REDUCED Attorney, Agent, or FirmOstrolenk, Faber, Gerb & LENGTH Soffen [75] Inventor: Isao Ueda, Tokyo, Japan [57] ABSTRACT [73] Asslgnee: Nlppon Electric Company Llmlted A multi-cavity klystron is provided which utilizes a Tokyo, Japan front stage wide band exclting cavlty group consisting [22] Filed: May 14, 1974 of one or more cavities and including at least one cavity coupled to an external reactance circuit. Means are [2]] Appl' 469758 provided for applying a voltage to accelerate the electron beam at least across the last drift tube interaction [52] US. Cl. 3l5/5.39; 315/534; 315/55 1; gap in said front stage wide band cavity group. A rear 315/552 stage bunching cavity group comprised of one or more [51] Int. Cl. H01J 25/10 cavities is tuned to a frequency of few percent greater [58] Field of Search 315/534, 5.35, 5.39, 5.43, than the center frequency operating band and an out- 315/5.44, 5.46, 5.51, 5.52 put cavity is provided to ultimately receive and pass the electron beam, said output cavity being tuned sub- [56] References Cited stantially to the center frequency of the operating UNITED STATES PATENTS band. By selecting a perviance measured across the 2,610,306 9/1952 Touraton et a1. 315/534 x Cathode and acceleratmg fl whmh greater 2,702,349 2,1955 McAnhur u 315/544 X than the values heretofore ut1l1zed, the perviances of 2 934 72 4 19 0 Pollack et aL 3 5 5 4 the succeeding gaps through the electron beam 2,960,658 11/1960 Dain et al. r 315/544 successively passcs y bc accordingly reduced to 3,4 56.207 7/1969 Badger 315/539 X alize a high conversion efficiency, typically of the 3,725,721 4/1973 Levin .1 315/543 order of 66.6%, without making the overall axial 3,775,635 11/1973 Faillon Ct al... 315/543 length of the klystron greater 3,811,065 5/1974 Lien 315/539 SIGNAL $OURCE- INPUT DRIFT TUBE 17 Claims, 2 Drawing Figures COLLECTOR) OUTPUT DRIFT TuaIsY 5 LoAoflzzy fi 4m INTERACTION GAP i 2e 4 OUTPUT CAVIfY-\Z/ I l-1'19?" N 3 DRIFT TuBE-/ 9\.

3rd CAVITY\/7\ 2nd DRIFT. Tuas /i,

CHOKEVM am INTERACTION 2nd cAvITY-IJ GAP m DRIFT Tues-'12 @I 6 a I 54: ll y $4 1% I:TI RAcTIm ACCELERATING ELECTRODE 79/ EXTERNAL 'REACTANCE ext /0 4 m INPUT m awry 7 I 2 2 CHOKE f2 HIGH-EFFICIENCY BROADBAND KLYSTRON AMPLIFIER OF REDUCED LENGTH BACKGROUND OF THE INVENTION The present invention relates to a wide band klystron. Heretofore, klystrons for use in amplification of television video signals have been required to have a broad bandwidth for amplification of the order of 68 MHz at 1 dB points in the frequency band for UHF television broadcasting (for instance, 470 MHz-89O MHz), and since this bandwidth exceeds 1% of a reference frequency, in such type of klystrons normally resonant frequencies of four to five cavities were staggered, and simultaneously therewith a perviance was selected at 1.5 to 3 X 10' pervs so that Q, of the re spective cavities upon operation, when a beam current is passed therethrough, may be lowered to a minimum by the loading of the electron beam.

In addition, in the UHF band the wavelength is longer than that in the GHz band, so that the construction in the cavity section of the klystron which is correlated to the wavelength is fairly large in size. Accordingly, a large perviance was also required for the purpose of avoiding an excessive tube length. On the other hand, however, a large perviance was accompanied with a shortcoming that an efficiency of conversion from an electron beam to a microwave circuit at the output cavity gap was lowered.

With reference to an article by WALDER and Mel- SAAC entitled Experimental Analysis of Biased Gap Klystron (IEEE Trans. ED, 1966), it is reported that when an accelerating bias was applied across the drift tube gap in the penultimate cavity, an efficiency of 66% was obtained. However, in this case, the wide band characteristics were out of the question. In addition, it is reported that when an accelerating voltage was applied across a drift tube gap in a second cavity, the effect of enhancing operating efficiency was not observed.

With reference to an article by T. G. MIHRAN entitled Design and Demonstration of a Klystron with 62 percent Efficiency (IEEE Trans. ED, Vol. ED-l8, No. 2, February 1971, P. 124-133), it is reported that when the bandwidth characteristics are disregarded, it is more preferable in view of an efficiency to select a perviance at about 0.5 X 10 perv.

OBJECTIVE OF THE INVENTION It is an object of the present invention to provide a multi-cavity klystron having a broad bandwidth, at high efficiency and a high gain and yet having its entire length shortened.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic structural view showing one preferred embodiment of the present invention, and

FIG. 2 is a schematic partial structural view showing another preferred embodiment of the present invention. 1

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1, reference numeral 2 designates a cathode, numeral 3 designates an accelerating electrode, numeral l designates a choke structure for insulating an input drift tube from a first cavity with respect to a DC. component, numeral 7 designates an external reactance circuit, numeral 9 designates an input cavity in a front stage wide band exciting cavity group, numeral 13 designates a second cavity in the same front stage cavity group, numeral 17 designates a third cavity in a rear stage bunching cavity group, numeral 20 designates an output cavity, numeral 11 designates a drift tube interaction gap in the first cavity, numeral 14' designates a choke structure for insulating a first drift tube from the second cavity with respect to a DC. component, numeral l5 designates a drift tube interaction gap in the second cavity, symbol E designates an accelerating voltage source for applying a voltage across the first cavity drift tube interaction gap, and symbol E designates an accelerating voltage source for applying a voltage across the second cavity drift tube interaction gap.

Now describing the present invention with reference to the accompanying drawings, in a schematic structural view of a four-cavity klystron shown in FIG. 1 which illustrates one preferred embodiment of the present invention, a perviance with respect to a cathode 2 and an accelerating electrode 3 is selected to lie in the range from 1.5 to 3 X 10 perv. and preferably to be a value of 2 X 10 perv. An input drift tube 4 whose downstream end forms a first interaction gap 11 jointly with the adjacent end of a first drift tube 12, is provided with a choke structure portion 10 which insulates the drift tube 4 from an input cavity 9 with respect to a DC. component but short-circuits the drift tube 4 thereto with respect to the microwave frequency, and said input drift tube 4 cooperates with the input cavity 9 to function as a conventional cavity resonator. With regard to the choke structure portion 10, a known choke structure in a microwave circuit is available, in which an insulator such as, for example, ceramics is interposed between the metallic members to hold them firmly and mechanically relative to one another, and which is as long as one fourth wavelength at the operating frequency of said klystron, and therefore it will not be described here in more detail.

An exciting signal applied from an external signal generator 5 is transmitted to the input cavity 9 having a loop 8, through a capacitor or an electro-static coupler 6 for intercepting a DC. component and a broad exciting circuit 7, to give a broad band velocity modulation to an electron beam 26 in the first drift tube interaction gap 11. The first drift tube 12 which defines the upper boundary of the first interaction gap 11, is insulated from a second cavity 13 with respect to a DC. component but short-circuited thereto with respect to a microwave frequency at a choke structure portion 14 as is the case with the input drift tube 4, although it is directly connected to the input cavity9, and thereby as a whole said drift tube 12 is constructed to serve as a part of both the cavity resonators 9 and 13.

The second cavity 13 is directly connected, in a conventional manner, to a second drift tube 16, a third cavity 17, a third drift tube 19, an output cavity 20 and an output drift tube 24, to form the remaining part of the klystron tube. The output cavity 20 is coupled to load 22 such as an antenna via, for instance, a loop 21. This output cavity 20 and the like are grounded similarly to the conventional structures. A collector 25 is normally grounded through an ammeter or the like, but it may be applied with a potential lower than the earth potential by means of a voltage source E in order to improve operating efl'iciency.

Assuming now that a conversion efi'iciency from the electron beam 26 to an external circuit 22 at an output cavity interaction gap 23 is 66.6%, that an equivalent perviance of the electron beam 26 at the same interaction gap is 0.8 X 10 perv. selected in the range from 0.3 to l X 10 prev., and that an output power of about 9.3W is fed to the external circuit 22, then it can be deduced that a DC. power of the electron beam across the fourth interaction gap or the output cavity interation gap 23 is 14 KW, and that the beam voltage and current are 12.5 KV and 1.12 A., respectively.

Since the perviance at the electron gun in the embodiment illustrated in FIG. 1 was selected at 2 X 10 perv. as assumed previously, the accelerating electrode voltage is 6.85 KV with respect to the cathode 2, and said voltage is applied by means ofa voltage source E In general, the input drift tube 4 is either shortcircuited to the accelerating electrode 3 upon manufacture or separately formed for the purpose of current measurement, but even in the latter case they are externally short-circuited so as to be at the same potential.

The difference voltage of 5.65 k\/ between the voltage of 12.5 kV and the voltage of 6.85 kV is divided into two voltages represented by voltage sources E and E, which are connected as shown in FIG. 1.

Thereby the electron beam is additionally accelerated by the voltage E across the first interaction gap 11, and further it is still additionally accelerated by the voltage E, across the second interaction gap 15.

Although the acceleration across the first interaction gap has a rather negative effect for the purpose of flattening the frequency response characteristics under the conditions of wide band excitation and heavy beam loading, in the embodiment illustrated in FIG. 1 the voltage of the voltage source E is selected to lie in the range from 0.5 to 3 KV and preferably at 1 kV in order to mitigate the requirement for withstand voltage at the choke portion 10 singly, to improve the focusing of the electron beam by progressively accelerating the same towards the collector as will be described later, and also to prevent feedback caused by backwardly travelling electrons.

Accordingly, in the illustrated embodiment, the equivalent perviance of the electron beam is varied from 2 X l" perv. to 1.62 X perv. across the first interaction gap, and if the voltage E across the second interaction gap is selected at 4.65 KV, then the equivalent perviance is varied from 1.62 X 10 perv. to 0.8 X 10 perv. across the second interaction gap.

Generally, a coupling coefficient between an electron beam and a cavity circuit across a drift tube interaction gap can be calculated by normalizing the structural dimensions with reference to a DC. velocity of the beam and an operating frequency. In addition, a density modulation developing within an electron beam travelling through a drift tube, is determined by means of a reduced plasma wavelength Aq, which is obtained by multiplying a plasma Wavelength that is in turn determined from beam voltage and current and a beam radius, by a configuration reduction factor that is determined from a drift tube diameter, a beam diameter, an operating frequency, etc.

Calculating on the basis of the dimensional and frequency data consisting of an operating frequency of 500 MHz, a drift tube radius of 9 mm, a beam radius of 6 mm, and a drift tube interaction gap distance (the same for every cavity) of 16 mm, then a coupling coefficient across the first interaction gap M,, a coupling coefficient across the second interaction gap M coupling coefficients across the third and fourth interaction gaps M M an effective plasma wavelength in the first drift tube A ql, and effective plasma wavelengths across the second and third drift tubes M2 M 3 are obtained as shown in the following TABLE-I. In addition, beam loading values Gb for the respective interaction gaps are also shown in the same table.

In the first and second interaction gaps 11 and 15, since a DC. accelerating voltace is applied across the opposed drift tubes, the velocity of the electron beam is raised upon passing across the interaction gap. Accordingly, though the coupling coefficient across the interaction gap is increased, the beam loading is reduced, as shown in Table I. In practice, the velocity changes gradually through the interaction gap, so that these factors would take intermediate values between the respective values calculated from the velocities at the front and rear edges of the interaction gap as indicated in the above table.

As described, according to the voltage distribution as illustrated in FIG. 1, the beam loading across the first interaction gap is about 2.5 times as high as the beam loading across the third and fourth interaction gaps, which implies that the electron beam loading can be made larger than the case where the initial equivalent perviance of the electron beam is selected at 0.8 X 10' perv., and that the Q-value of the input cavity can be lowered accordingly. 7

Under such a low-Q condition, owing to the function of the input cavity 9 and the filter circuit 7 coupled thereto, the velocity modulation imposed upon the electron beam 26 by the signal transmitted from the signal generator 5 in the first interaction gap 11 wouldv have a very broad band-width.

The same electron beam 26 is subjected to density modulation as it travels through the subsequent first drift tube 12. At the stage of a lower level of microwave signal, it is a common. practice to use an electrically longer drift tube length in order to assure a gain to a certain extent. Preselecting now this value at L, 0.2 MI, then L, 17 cm is obtained because Aql is as short as 84.5 cm as shown in TABLE-I, and thus this length can be extremely shortened in contrast to the value of 29 cm when the equivalent perviance is initially selected at 0.8 X 10 perv.

For the purpose of enhancing the electron current conversion efficiency, it is necessary to detune the second and third cavities by a few percent, i.e., 1 to 5 percent, towards, a higher frequency, and to increase the bunching density in the electron beam by progressively shortening the drift distance in such a manner that the second drift tube length is selected at L 0.1 M12 17 cm and the third drift tube length is selected at L 0.09 M13 13 cm. However, if the perviance of the electron beam in the third drift tube region is 2 X 10 perv., then the drift tube length takes an extremely short value of L 0.09 At 6.3 cm, resulting in disadvantages-that there remains no space between the adjacent cavities for accommodating a cooling structure (i.e., radiating pins 26 FIG. 1) and that the drift tube interaction gap must be positioned very close to the cavity wall and thereby the impedance is necessarily lowered.

Whereas, according to the present invention, since the equivalent perviance is lowered to-().3 to l X 10" perv. in the second and third drift tube regions where the bunc hing in the electron beam becomes substantial, bunching having a small velocity dispersion and a high conversion efficieney can be realized. 7

As described, in the klystron according to the present invention, in the input cavity and the first drift tube region which are lower signal level regions, a wide band excitation is applied to the electron beam under the state having a large perviance, that is, having a low Q, value to assure a sufficient gain in a mechanically short distance, while in the second and subsequent cavities which largely affect the efficieney, by reducing the equivalent perviance the velocity dispersion of the electron beam is suppressed, the restriction'for the distance between the adjacent cavities is mitigated and the efficieney is fully enhanced. Since the resonant points of the second and third cavities are shifted to a higher frequency outside of the operating frequency band in order to enhance operating efficieney, these cavities would not degrade the wide band characteristics at the preceding stage.

Thus, in the multi-cavity klystron according to the present invention, the distance from the first interaction gap to the fourth interaction gap takes a value of 0.2 M11 0.115 M12 0.09 M13 47 mm in contrast to the corresponding distance in the prior art of 0.405 M13 59 cm, so that a high gain may be easily assured with a tube body that is shorter than the prior art tubes by more than 10 cm, and yet a wide band characteristic and an efficieney of 60% or higher can be assured similarly to the prior art tubes.

In the first and second interaction gaps where an accelerating voltage is applied across the drift tube interaction gap, the electron beam is focused by an electrostatic lens effect, and so the magnetic focusing becomes easier. In cases where it is desired to improve the efficieney by lowering the collector potential, sometimes the backwardly travelling electrons which have been accelerated by the inverse voltage E and thus travel backwardly, from the collector region, feed back energy to the input cavity while repeating interaction with the microwave circuit in the drift tube interaction gap in an opposite sequence to the principal beam, resulting in various unstable phenomena. However, in the klystron according to the present invention, provided that the voltage values of the voltage sources satisfy the condition of E, E the backwardly travelling electrons would enter the drift tubes and the like before they return to the first interaction gap, so that the faults caused by the backwardly travelling electrons would not occur in practice. 7

Still further, the voltage at the accelerating electrode that is opposed to the cathode, which hasa high operating temperature, is of a low voltage value'because the perviance is selected at a relatively high value, and accordingly, there is an advantage that the occurrence of sparking accidents are extremely reduced.

Obviously, the choke portion IOof the input cavity 9 and the choke portion 14 of the second cavity 13 must have withstand voltages of, for instance, 1 kV or higher and 4.65 kV or higher, respectively.

Although a multi-cavity klystron involves only a few problems relating to safety due to the presence of high voltage because it is generally used as enclosed in a magnetic focusing device, it is desirable to connect the first and second cavities to a structure having a potential that is close to the earth potential as shown in FIG.

In case of applying voltages across the first and second interaction gaps, the structure of insulating with respect to a DC. but short-circuiting with respect to microwave frequencies can be relatively easily practices because the associated cavities are cavities supporting a low power level of microwave energy.

While the drift tube portions are formed into a choke structure in the embodiment illustrated in FIG. 1, the invention is not limited thereto, but various alternative modes of the preferred embodiment can be adapted such that the choke structure may be formed in the upper and lower cavity wall portions.

It is preferable for safety reasons to form the input cavity and the wide band circuit integrally so as to be capable ofbeing maintained at the same potential.

The connection of the voltage source circuit in FIG. 1 is illustrative, and the invention is not limited thereto. Since the accelerating electrode current and the drift tube currents are small relative to the principal beam current, the voltage source connection can be made in such manner that a bleeder resistance is connected across a single voltage source of a voltage equal to E E E and the necessary potentials are applied from the tap points of said bleeder resistance to the drift tubes and the like. I

The lowering of the collector potential by means of the voltage source E has been illustrated to show the associated feature of the klystron according to the present invention, and any known techniques for lowering the collector potential may be practiced without deteriorating the effect and advantage of the present invention. The present invention could be embodied not only in an internal cavity type but also in an external cavity type.

While an embodiment in'which accelerating voltages are applied across the first and second interaction gaps l1 and 15, respectively, has been illustrated in FIG. I, an alternative embodiment may be utilized in which the accelerating voltage is not applied across either the first or the second interaction gap and the perviance is kept high so as to have a wide band characteristic, but the electron beam is abruptly accelerated across the third interaction gap where a highly efficient bunching is achieved' FIG. 2 shows another embodiment of the present invention, in which the perviance is set at a relatively high value, whereby a low voltage E is applied to an input drift tube 30 formed integrally with an accelerating electrode and is further applied to a first drift tube 38 via a lead wire 40 insulated by an insulator 39, said drift tubes 30 and 38 being insulated with respect to a DC. component from cavities 31 and 43, respectively, of an internal cavity type of klystron but short-circuited to parts 34 and 41, respectively, through a choke portion with respect to microwave frequencies.

A signal is applied to the input cavity 31 from a signal generator 37 via an auxiliary cavity 36 and an input loop 35, and thereby a wide-band velocity modulation is imposed upon the electron beam 26 across the first interaction gap 33 under a state of heavy beam loading.

The second cavity 43 is further coupled to a matched load 47 via a loop 45 through a wide-band circuit 46 so that the impedance as viewed from the second interaction gap may have a wide-band and low-Q characteristic Accordingly, a wide-band velocity modulation is imposed upon the electron beam also across the second interaction gap. and simultaneously therewith an acceleration corresponding to a DC. voltage E is achieved there-across. In a second drift tube 48 and subsequent circuits. the electron beam becomes a beam having a low perviance so that bunching having a high conversion efficiency may be achieved.

According to the embodiment shown in FIG. 2, be-

cause of the fact that the cavity walls are entirely maintained at earth potential, there is no need to insulate with respect to a DC. level, the variable frequency O means 32 and 44, the exciting input circuits 36 and 37, and the second cavity loading circuits 46 and 47 from the cavity walls, and so this construction is safe and easy to practice.

In FIG. 1, the values of the perviance at the electron gun section. the equivalent perviance of the electron beam in the front stage cavity section where a wideband velocity modulation is achieved with a low level of signal, and the equivalent perviance of the same in the rear stage cavity section where highly efficient bunching with a high level of signal and extraction of an output power. are merely illustrative, and the scope of the present invention should not be limited thereto.

What is claimed is:

l. A multi-cavity klystron having spaced apart electron beam generating means and beam collection means. said klystron having apparatus positioned between said beam generating and collection means comprising:

a front stage wide band exciting cavity group comprising a plurality of axially aligned spaced apart drift tubes for passing the electron beam therethrough and a plurality of cavities, each of said cavities spanning selected pairs of said axially aligned drift tubes;

at least one of said cavities having an external reactance circuit coupled thereto for enhancing wide band response of the klystron;

means for applying a voltage to a selected one of said drift tubes for accelerating an electron beam at least across the last drift tube interaction gap in said front stage wide band cavity group;

a rear stage bunching cavity group comprising at least one cavity spanning a selected pair of said drift tubes and being tuned to a frequency one to a few percent higher than the center frequency of the operating band;

an output cavity spanning another selected pair of said drift tubes and being tuned substantially to the center frequency of said band and having an interaction gap forextracting energy from said electron beam;

choke means positioned between that one of the drift tubes forming the aforementioned interaction gap for accelerating the electron beam which is positioned closest to said beam generating means and the cavity associated therewith for isolating the drift tube closest to the beam generating means from said associated cavity to decouple D.C. components from the aforesaid drift tube.

2. A multi-cavity klystron comprising a cathode for developing an electron beam;

an accelerating electrode and a voltage source coupled thereto for accelerating the electron beam produced by said cathode;

input and first spaced drift tubes axially arranged to receive and pass said electron beam therethrough, the spacing between said input and first drift tubes forming a first interaction gap;

an input cavity spanning said first interaction gap;

choke means coupled between said input drift tube and said cavity;

an external reactance circuit coupled to said first cavity for broadening the operating bandwidth;

second and third spaced drift tubes being axially arranged to receive and pass said electron beam therethrough;

said second and third drift tubes being spaced from said first drift tube and from each other to form second and third interaction gaps:

a second cavity spanning said second gap and being electrically connected to said second and third drift tubes, said second cavity being tuned to a frequency deviating by a few percent from the center frequency of the operating band;

a fourth drift tube being axially arranged to pass said electron beam therethrough and to form with said third drift tube a fourth interaction gap;

an output cavity spanning said fourth gap and being electrically connected to said third and fourth drift tubes, said output cavity being tuned to the center frequency of the klystron;

an external circuit coupled to said output cavity for receiving the microwave energy developed by said klystron at a high efficiency level.

3. The klystron device of claim 2 wherein the perviance with respect to said cathode and accelerating electrode is greater than the perviance across the first interaction gap.

4. The klystron device of claim 3 wherein the perviance of said second gap is less than the perviance of said first gap.

5. The klystron device of claim 4 wherein the perviance of said third gap is less than the perviance of said second gap.

6. The klystron of claim 2 further comprising a cooling structure;

the perviance of said third gap being reduced by an amount sufficient to increase the distance between said third and output cavities to facilitate the placement of said cooling structure therebetween.

7. The klystron of claim 2 further comprising a collector axially arranged to intercept the electron beam as it passes beyond the fourth drift tube;

second DC voltage means for reducing the DC. voltage level of said collector relative to said fourth drift tube.

8. The klystron of claim 2 further comprising means for coupling a DC. accelerating voltage across said first gap.

9. The klystron of claim 8 further comprising means for coupling a DC. accelerating voltage across said second gap.

10. The klystron of claim 9 further comprising a collector axially arranged to intercept the electron beam as it passes beyond the fourth drift tube:

second DC. voltage means for reducing the D.C.

voltage level of said collector relative to said fourth drift tube.

11. The klystron of claim 10 wherein the DC. voltage coupled across said second gap is at least equal to the DC. voltage coupled between said fourth drift tube and said collector.

12. The klystron of claim 2 further comprising a matched load coupled to said second cavity so that the impedance as viewed from the second gap is wide-band and has a low-Q characteristic.

13. The klystron of claim 2 wherein the second cavity is tuned to a frequency greater than the center frequency of the operating band.

14. The klystron of claim 5 wherein the perviance of said third gap is in the range from 0.3 to 1.0 X 10 perv.

15. The klystron device of claim 2 wherein the cou pling coefficient M of said first interaction gap is in the range of 0.891 M 0.905.

16. The klystron device of claim 15 wherein the coupling coefficient M of said second interaction gap is in the range 0.905 M 0.945.

17. The klystron device of claim 16 wherein the coupling coefficient M of the third interaction gap is at least equal to the largest value of M l l l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,994,917 Dated September 9, 1975 Inventorka I SAO UEDA It is certified that error appears in the above-identified patent Q and that said Letters Patent are hereby corrected as shown below:

On the cover page of the Patent, left-hand column, after Item "[21]" and before Item "[52]" kindly insert Foreign Application Priority Data May 24, 1973 Japan. .58,446

' C Signed and Scaled this seventeenth D 3} Of February 1 976 [SEAL] Arrest:

RUTH C. MASON I C. MARSHALL DANN Attesling Officer (ummissinner oj'Palents and Trademarks 

1. A multi-cavity klystron having spaced apart electron beam generating means and beam collection means, said klystron having apparatus positioned between said beam generating and collection means comprising: a front stage wide band exciting cavity group comprising a plurality of axially aligned spaced apart drift tubes for passing the electron beam therethrough and a plurality of cavities, each of said cavities spanning selected pairs of said axially aligned drift tubes; at least one of said cavities having an external reactance circuit coupled thereto for enhancing wide band response of the klystron; means for applying a voltage to a selected one of said drift tubes for accelerating an electron beam at least across the last drift tube interaction gap in said front stage wide band cavity group; a rear stage bunching cavity group comprising at least one cavity spanning a selected pair of said drift tubes and being tuned to a frequency one to a few percent higher than the center frequency of the operating band; an output cavity spanning another selected pair of said drift tubes and being tuned substantially to the center frequency of said band and having an interaction gap for extracting energy from said electron beam; choke means positioned between that one of the drift tubes forming the aforementioned interaction gap for accelerating the electron beam which is positioned closest to said beam generating means and the cavity associated therewith for isolating the drift tube closest to the beam generating means from said associated cavity to decouple D.C. components from the aforesaid drift tube.
 2. A multi-cavity klystron comprising a cathode for developing an electron beam; an accelerating electrode and a voltage source coupled thereto for accelerating the electron beam produced by said cathode; input and first spaced drift tubes axially arranged to receive and pass said electron beam therethrough, the spacing between said input and first drift tubes forming a first interaction gap; an input cavity spanning said first interaction gap; choke means coupled between said input drift tube and said cavity; an external reactance circuit coupled to said first cavity for broadening the operating bandwidth; second and third spaced drift tubes being axially arranged to receive and pass said electron beam therethrough; said second and third drift tubes being spaced from said first drift tube and from each other to form second and third interaction Gaps: a second cavity spanning said second gap and being electrically connected to said second and third drift tubes, said second cavity being tuned to a frequency deviating by a few percent from the center frequency of the operating band; a fourth drift tube being axially arranged to pass said electron beam therethrough and to form with said third drift tube a fourth interaction gap; an output cavity spanning said fourth gap and being electrically connected to said third and fourth drift tubes, said output cavity being tuned to the center frequency of the klystron; an external circuit coupled to said output cavity for receiving the microwave energy developed by said klystron at a high efficiency level.
 3. The klystron device of claim 2 wherein the perviance with respect to said cathode and accelerating electrode is greater than the perviance across the first interaction gap.
 4. The klystron device of claim 3 wherein the perviance of said second gap is less than the perviance of said first gap.
 5. The klystron device of claim 4 wherein the perviance of said third gap is less than the perviance of said second gap.
 6. The klystron of claim 2 further comprising a cooling structure; the perviance of said third gap being reduced by an amount sufficient to increase the distance between said third and output cavities to facilitate the placement of said cooling structure therebetween.
 7. The klystron of claim 2 further comprising a collector axially arranged to intercept the electron beam as it passes beyond the fourth drift tube; second D.C. voltage means for reducing the D.C. voltage level of said collector relative to said fourth drift tube.
 8. The klystron of claim 2 further comprising means for coupling a D.C. accelerating voltage across said first gap.
 9. The klystron of claim 8 further comprising means for coupling a D.C. accelerating voltage across said second gap.
 10. The klystron of claim 9 further comprising a collector axially arranged to intercept the electron beam as it passes beyond the fourth drift tube; second D.C. voltage means for reducing the D.C. voltage level of said collector relative to said fourth drift tube.
 11. The klystron of claim 10 wherein the D.C. voltage coupled across said second gap is at least equal to the D.C. voltage coupled between said fourth drift tube and said collector.
 12. The klystron of claim 2 further comprising a matched load coupled to said second cavity so that the impedance as viewed from the second gap is wide-band and has a low-Q characteristic.
 13. The klystron of claim 2 wherein the second cavity is tuned to a frequency greater than the center frequency of the operating band.
 14. The klystron of claim 5 wherein the perviance of said third gap is in the range from 0.3 to 1.0 X 10 6 perv.
 15. The klystron device of claim 2 wherein the coupling coefficient M1 of said first interaction gap is in the range of 0.891 < M1 < 0.905.
 16. The klystron device of claim 15 wherein the coupling coefficient M2 of said second interaction gap is in the range 0.905 < M2 < 0.945.
 17. The klystron device of claim 16 wherein the coupling coefficient M3 of the third interaction gap is at least equal to the largest value of M2. 